Its a similar reason for the MBC.
Since the car uses a BPC(boost pressure controller), like a mbc but controlled by the ECU. If the ECU is telling the BPC to allow an X amount of boost pressure, but you have your MBC set to some other amount, the engine management (T7) is getting different MAP readings while thinking the boost pressure is set by the BPC, when its not.
In layman terms....
Yeah, T5.2 its -.3 better then the rest.
1989 900 SPG: Trionic 5(stg1), Aero seats, TD04-15G, 3" Turboback. SOLD
1993 9000 Aero: 3"Turboback, T5Suite Stg.3